Japanese unexamined patent application No. JP-A-2003-295562 describes a related art image forming apparatus such as a color printer. The related art image forming apparatus includes a plurality of process cartridges each having a photosensitive drum and a plurality of developer supply units each containing developer to be supplied to the process cartridge. The process cartridge and the developer supply unit can be attached to and detached from a printer main body along the same direction. The developer contained in each of the developer supply units is supplied from a developer discharge port to a developer reception port of the corresponding process cartridge.